New types of data services such as the packet data service GPRS (General Packet Radio Service) are currently being introduced into existing mobile networks based on the GSM standard. Transmission in the mobile network does not take place in a connection-oriented or circuit-switched manner, but in the form of packet data. This type of transmission more effectively utilizes the given transmission resources in the mobile network. The architecture for the packet data service assumes (see also FIG. 1) that the fixed or mobile data terminal or communications terminal used by the subscriber, e.g. a mobile station, is served at its respective location by a service network node (SGSN: Serving GPRS Support Node). Access to a packet data network is required in order to receive or transmit packet data. For this purpose, access network nodes (GGSN: Gateway GPRS Support Nodes) are provided, which in each case implement the access to the packet data network, e.g. the Internet, and support a specific packet data protocol e.g. the Internet Protocol (IP). A tunnel, via which the packets are transmitted, is provided in the mobile network between the service network node and the access network node.
Particularly for third-generation communications systems, e.g. the UMTS (Universal Mobile Telecommunication System), an architecture is provided for IP-based mobile networks which has an extension of the existing GPRS architecture in order to provide the call control components. One of the fundamental new components for this purpose is a Call State Control Function (CSCF). This function performs, inter alia, the call control functions previously performed by Mobile service Switching Centers (MSC). The Call State Control Function CSCF therefore also serves in particular as a connection control instance for multimedia services in the packet-switching or IP-based core network. The Call State Control Function CSCF which is responsible for a call is determined on a data-terminal-specific or subscriber-specific basis, i.e. it is defined by the Home Public Land Mobile Network HPLMN of the subscriber.
Registration of data terminals or mobile subscriber stations on the registration device, e.g. of the packet data network of the 3GPP standard (3GPP: 3rd Generation Partnership Project) which is currently being set up, is carried out in stages in two instances of the network. The first instance is the packet data service GPRS, whereby the procedures for the 3GPP system (GPRS Attach and PDP Context Activation) are already described and standardized. The second instance is the Call State Control Function. To register with the Call State Control Function CSCF, the subscriber must be authenticated and authorized, i.e. its identity and access authorization(s) are checked.
However, the current standard does not yet provide procedures for selection of the Call State Control Function CSCF or the authorization and authentication of a subscriber or data station communicating via any given IP protocol, e.g. SIP (Session Initiation Protocol) on the Call State Control Function CSCF, i.e. for registering a subscriber and checking its authorization. However, these procedures are absolutely essential for secure operation of e.g. packet-oriented 3GPP systems.